The use of femtosecond lasers to produce permanent changes in the refractive index of various glasses has been described in the scientific literature. Index modified glass materials can be formed into, e.g., waveguides, couplers, and diffraction gratings. U.S. Pat. No. 5,761,111 (Glezer) describes a femtosecond laser process for producing two- and three-dimensional optical storage in glasses by producing regions containing volume elements (voxels) that range from submicron to a few microns in diameter with lengths in the range of a few microns to millimeters. U.S. Pat. No. 5,978,538 (Miura) describes a femtosecond laser process for forming optical waveguides in oxide, halide, and chalcogenide glasses. WO 01/09899 describes a femtosecond laser process for direct writing of optical devices in soft, silica-based glasses.
In Kondo, et al., Optics Letters, Vol. 24, p. 646, the authors describe fabricating long-period fiber gratings by using focused irradiation of infrared femtosecond laser pulses. The authors initially attempted to write through the polymeric coating, but found that even when the laser power was reduced to such a degree that no index changes in the core occurred, the acrylate resin was ablated.